1. Field of the Invention
This invention relates to a transmission apparatus, which utilizes a plastic fiber, for use in an optical communication system, and the like.
2. Description of the Related Art
Ordinarily, as light propagation paths in optical communication, single mode fibers or multi-mode fibers containing quartz glass as a principal material are utilized. The single mode fibers or the multi-mode fibers containing quartz glass as a principal material have a diameter of at most 200 μm. In alignment processes for the fibers, a high position matching accuracy on the order of micron is required. Therefore, fiber laying operations under ordinary environmental conditions, such as conditions at construction work sites, are not easy to perform. The difficulty of the fiber laying operations obstructs further popularization of the fibers described above.
Recently, plastic fibers, which have a comparatively large diameter and are comparatively easy to lay, have been proposed. However, for reasons of production processes, the plastic fibers primarily have a step index type of structure. The step index types of plastic fibers cannot transmit a signal of a high bit rate over a long distance. Specifically, in cases where a pulsed light signal is inputted into an entry end face of the step index type of fiber, a phenomenon is encountered in that a wave form of the pulsed light signal becomes deformed and spreads at a radiating end face of the fiber after being propagated over a long distance through the fiber. Therefore, in cases where a successively pulsed light signal is transmitted through the step index type of fiber, the problems occur in that the pulses, which are adjacent to each other on the time axis, overlap one upon the other, and a perfectly extinct state cannot be obtained with respect to a logic “0” level of the signal at the radiating end face of the fiber. In other words, in cases where the pulsed light signal of a short pulse width is transmitted through the fiber, a logic “0” level and a logic “1” level of the signal cannot easily be discriminated from each other after the signal has been transmitted through the fiber. Accordingly, the step index types of plastic fibers are not appropriate for large capacities of optical communication. The problems described above are described in, for example, “Fundamentals and Practice of Plastic Optical Fibers” by Yasuhiro Koike, supervised by Seizo Miyata, NTS K. K., pp. 84-87, 2000.
In order for the aforesaid problems to be eliminated, a graded index type of fiber, which has a comparatively large diameter and is free from an increase in pulse width of a pulsed light signal after being transmitted, has been proposed and is expected to be used in practice. However, it has been found that the problems described below are encountered with this type of fiber.
Specifically, a plastic fiber using a fluoride (e.g., Lucina, supplied by Asahi Glass Co., Ltd.) has been used in practice. However, the fluoride raw material is expensive, and therefore the cost of the fiber cannot be kept low. Also, in cases where the core diameter of the fiber is set at a large value, since the amount of the fluoride becomes large, the cost becomes high. Therefore, with the plastic fiber using a fluoride, it is not possible to utilize the advantages of the plastic fiber in that the cost is capable of being kept low, and in that the fiber laying operations are easy to perform by virtue of a large-diameter core.
As a core material, which is available at a low cost and is capable of easily forming a large-diameter core, a polymethyl methacrylate (PMMA) has heretofore been known. It may be considered that a graded index type of optical fiber is produced by the utilization of the PMMA as a principal core material. However, as illustrated in FIG. 6, the fiber provided with the core, which contains the PMMA as a principal constituent, has the characteristics such that the wavelength regions associated with a low propagation loss occur only at limited wavelength regions (in the vicinities of 520 nm, 570 nm, and 650 nm) falling within the visible region. (FIG. 6 is cited from a literature “POF-polymer optical fibers for data communication,” Springer-Verlag, 2002.)
Of the aforesaid limited wavelength regions associated with a low propagation loss, a wavelength region of light, which a semiconductor laser or a light emitting diode (LED) allowing quick modulation is capable of producing, is currently only the region in the vicinity of 650 nm. As for the production of light having wavelengths falling within a wavelength region shorter than 650 nm, research has heretofore been conducted to develop a laser utilizing a II-VI Group compound semiconductor. However, a laser utilizing a II-VI Group compound semiconductor, which laser is capable of producing light having wavelengths falling within the wavelength region shorter than 650 nm and has practically acceptable reliability, has not yet been obtained.
Of the wavelength region in the vicinity of 650 nm, which wavelength region is associated with a low propagation loss, the propagation loss characteristics with respect to a wavelength region of 630 nm to 680 nm are such that the propagation loss is as low as approximately at most 300 dB/km. In particular, the propagation loss characteristics with respect to a narrow wavelength region of 640 nm to 660 nm are such that the propagation loss is as low as approximately at most 200 dB/km. In cases where light having wavelengths outside the aforesaid narrow wavelength region, e.g. wavelengths slightly longer than 660 nm, is to be propagated, the propagation loss varies markedly in accordance with the wavelengths. Therefore, in such cases, if the wavelengths of the light produced by a light source alter, the characteristics of the transmission apparatus will vary. It may be possible that the transmission characteristics exhibiting large variance be improved by the utilization of a new material for the fiber core. However, in such cases, the cost of the fiber will become higher than the cost of the fiber utilizing the ordinary PMMA.
As a light source capable of producing light having wavelengths in the vicinity of 650 nm, which light source is capable of producing the light having an intensity of approximately 1 mW necessary for optical communication and allows quick modulation ranging from 400 MHz to at least 1 GHz, an end face emission type of semiconductor laser, which is currently utilized for DVD's, and the like, is most excellent for high stability of laser beam production at high temperatures and for high reliability. However, in cases where the end face emission type of the semiconductor laser is utilized for optical transmission in combination with the plastic fiber, the problems of the Fabry-Pérot resonator types of lasers occur in that the wavelength of the produced laser beam shifts in accordance with temperatures. The problems described above occur due to the characteristics of the semiconductor such that the energy gap of the semiconductor is dependent upon temperature. The shift of the wavelength of the produced laser beam is represented by the formula shown below.             Δλ      0              Δ      ⁢                           ⁢              T        j              =            -              1.24                  E          g          2                      ·                  ⅆ                  E          g                            ⅆ                  T          j                    wherein λo represents the wavelength of the produced laser beam, Tj represents the bonding temperature of the semiconductor laser, and Eg represents the energy gap.
In the cases of the 650 nm-band semiconductor laser, the actual dependence of the wavelength upon temperature is in accordance with 0.2 nm/deg. Therefore, in cases where the ambient temperature rises by 100° C., the wavelength of the produced laser beam shifts by approximately 20 nm toward the longer wavelength side. Also, a variation of the absolute wavelength at the time of the production of the semiconductor laser is approximately ±5 nm. Therefore, in cases where the shift described above and the variation described above are summed up, it is necessary for the wavelength fluctuation range of approximately 30 nm to be taken into consideration. However, in such cases, with the plastic fiber having the propagation characteristics illustrated in FIG. 6, since there is the risk of the propagation loss becoming large, the problems occur in that, for example, limitation is imposed upon the length of distance over which the plastic fiber is utilized.
The problems described above may be solved with a technique where in a peltier device, which is employed for optical communication through trunk lines, is utilized to keep the temperature at a predetermined value through heating or cooling. However, with the technique utilizing the Peltier device, the problems occur in that, since the cost of the Peltier device is high, the cost of the transmission apparatus cannot be kept low.